Small crack in screed but cant change level of floor what can we do

[The Legend; Phil Hobson RIP]

I am always wary of screed mixed on site, ratios are often wrong, although it can be done correctly I prefare ready mix when possible.

 

[jwatson]

would a semi dry screed cope any better with the shrinkage as no where near as much water, and would it help limit secondary effects a long as compacted properly???

When you add water to cement in a sand cement screed it undergoes two basic mechanisms which cause it to shrink. The first is that water evaporates and the second is the apid hydration of the cement. These cause the screed to shrink in its early life. This effect is called primary shrinkage. Coupled with lack of compaction which leads to intrinsic weakness in the screed these are the primary causes of early age cracking in cement based materials.

Once the screed has set and become rigid and has ostensibly dried there will always be some moisture present and as long as this is there the hydration reaction continues. This hydration coupled with carbonation of the screed means tha for very long periods (years and years in some instances) the screed continues to shrink very slowly. This causes further cracking over time. This is called secondary shrinkage.

The ways to minimise this sort of cracking are to ensure that the screed is cured properly initially, ensure that it is fully compacted and does not dry too rapidly. This way it can build up enough internal strength to overcome the effects of the shrinkage. Lastly it must be remembered that this secondary shrinkage will always occur regardless of you efforts so correct screed design ais also essential I.e. joints in the right places and bay sizes not over sized.

Polypropylene fibres can also help but these deal mostly with primary shrinkage.


Hope this helps although much oversimplified.......

 

[Diamond Pool Finishers]

original screed IS SEMI-DRY, WHEN MIXED PROPERLY, and if LAID PROPERLY TO BS ,It SHOULD BE WELL COMPACTED , there are so many that think they can screed :yikes: ,that just pick-up the basics, ( builders ) then balls it -up, AJAKS must see lots of these jobs, i even look on line and see so-called pros talking rubbish,in the instructions :smilewinkgrin: ah well you get what you pay for :lol:

 

[Ajax123]

Surely using rebar in the screed is the best policy and avoids the majority of these sorts of cracking issues??

Sand cement screeds should be reinforced using either d49 steel mesh or polypropylene fibres with the latter being the more popular. These are only to help with plastic shrinkage cracking which is primary shrinkage. They will help to stabilise against long term cracking but will not prevent it entirely. The way to avoid shrinkage racking would be to not use cement I guess.

 

[Diamond Pool Finishers]

how would the argument for a very week mix be , we have laid screed as week as 1-5 1-6 , this helps with less cracking,or what about a bit of lime ? AJAX

 

[Diamond Pool Finishers]

how would the argument for a very week mix be , we have laid screed as week as 1-5 1-6 , this helps with less cracking,or what about a bit of lime ? AJAX

For domestic houses the above .
we only use polymer -modified screeds in the swimming pools we do now 1-3 strength

 

[Spearce]

thank you all for this just got back from work and this is of great help. So to summarise at this stage the thinest matting I can find is best. Can the tile height be lost over the room size when the tiles laid. Or will we have to have a step. Or do we get the builder to do something else. I wish we could get the floor ripped up and started by somebody that knows what they are doing. We feel gutted as we trusted this builder who said he could do it.

 

[Spearce]

just got back from work and some really good stuff from you all. thanks so much. To summarise. we now need to put down some sort of matting down. The thinner the better. If the height of this is to much for the exsisting flooring can this height be lost over the size of the floor. 6m x 4.5m. Wish we could get the floor ripped up and started again. We trusted the builder as he said he could do the job but know wish we got somebody that knew what they were actually doing. thanks again.

 

[Ajax123]

original screed IS SEMI-DRY, WHEN MIXED PROPERLY, and if LAID PROPERLY TO BS ,It SHOULD BE WELL COMPACTED , there are so many that think they can screed :yikes: ,that just pick-up the basics, ( builders ) then balls it -up, AJAKS must see lots of these jobs, i even look on line and see so-called pros talking rubbish,in the instructions :smilewinkgrin: ah well you get what you pay for :lol:

Yep......

 

[Ajax123]

how would the argument for a very week mix be , we have laid screed as week as 1-5 1-6 , this helps with less cracking,or what about a bit of lime ? AJAX

This becomes a trade off between strength and malleability. Malleability should ot be confused with flexibility. Old fashioned lime screeds were very weak essentially and they moved like billyo without cracking...Portland cement has a lot to answer for......

Pas long as thereis enough cement to coat the aggregate particles with cement paste so that it holds them together then the water cement ratio can be very low. Typically a water cement ratio of just 0.2 will give you a strong screed but unfortunately you would never compact it.

 

[Ajax123]

just got back from work and some really good stuff from you all. thanks so much. To summarise. we now need to put down some sort of matting down. The thinner the better. If the height of this is to much for the exsisting flooring can this height be lost over the size of the floor. 6m x 4.5m. Wish we could get the floor ripped up and started again. We trusted the builder as he said he could do the job but know wish we got somebody that knew what they were actually doing. thanks again.

NO need to rip it up. The crack can be repaired by using a simpl low viscosity epoxy resin to fillet. Then use an uncoupling membrane before tiling. Jobs a goodun

 

[Tim@Larsen]

Just to add my tuppence worth, I agree with everything Ajax has said. There should be no reason to rip the screed out if it is sound. However, it is important to be sure that it has 'finished' cracking before carrying out the crack injection. If you repair the crack too early you may find another crack appearing elswhere in the floor.

Once the floor has been repaired there should be no issue tiling onto it, but a decoupling mat would be belt and braces to be safe.

Cement based screeds are perfectly ok for use with underfloor heating, the problem is that the screed is typically around 75-100mm deep and so takes a long time to fully dry out, if the heat is turned on too early the screed is force-dried and they don't have the same tensile strength as gypsum screeds so are much more at risk of cracking. That said, it does seem like your builder was trying to do thngs right!